An organic electroluminescent element emitting white light (hereinafter referred to as organic element emitting white light) has been expected to go into various devices, e.g., thin display devices, illuminator for display devices and illuminator.
Patent Document 1 discloses a structure of organic element emitting white light. It has two light-emitting layers, one emitting a blue color on the anode side and the other a yellow to red color on the cathode side, where each layer contains a dopant which determines color of emitted light. It discloses a 10 nm thick layer for emitting a blue color and 40 nm thick layer for emitting a yellow to red color in EXAMPLE 1, suggesting a low hole transport capacity of the material for the blue-emitting layer.
A material for a light-emitting layer generally has a high hole transport or electron transport capacity. Therefore, electrons and holes are combined with each other at the light-emitting end on the electrode side, in which a carrier of low hole transport capacity is injected.
Patent Document 2 discloses an organic electroluminescent element comprising a mixed layer of hole transport material, electron transport material and dopant as a light-emitting layer. In this light-emitting layer, each of holes and electrons move in a material of high transport capacity, to control its degradation. The document also discloses a structure of organic element emitting white light, where the transport layer adjacent to the light-emitting layer is incorporated with a dopant to achieve light of white light spectral pattern by the light-emitting layer and dopant-containing transport layer.    Patent Document 1: JP-A 2003-272857    Patent Document 2: JP-A 2002-043063